This invention relates to an evaporator apparatus, especially for air-conditioning units of motor vehicles, which includes an evaporator assembly having several evaporator pipes, and a coolant feeding system comprising an expansion valve, a flow divider for dividing the coolant flow, and connecting means such as a pipe, which connects the expansion valve to the flow divider and which has at least one bend.
Evaporator apparatuses of this type are known in principle. They include a thermostatically controlled expansion valve that is supplied with coolant. A flow divider is arranged downstream of the expansion valve to distribute the coolant flow evenly to various sets (i.e., series connected groups) of evaporator pipes. The actual evaporation then takes place in the evaporator assembly.
The heat exchange surface of the evaporator assembly will be used optimally only when the coolant evaporates completely at the end of all parallel evaporator pipe sets and is overheated by an amount that is equally large in all sets. This overheating is used as the regulating variable for the control of the coolant flow by one or several thermostatic expansion valves.
The flow divider is designed so as to evenly distribute the coolant flow to the different sets of evaporator pipes. In a conventional embodiment, a Venturi distributor is used which divides the coolant flow to corresponding circular segments (see U.S. Pat. No. 2,803,116). To ensure that the coolant is admitted uniformly to the various sets of evaporator pipes, it is essential that a homogeneous two-phase (liquid/gas) flow exists at the inlet of the flow divider. If the flow is asymmetrical or non-homogeneous, different amounts of coolant are admitted to each set of pipes which impairs the efficiency of the evaporator and, under certain circumstances, may also result in unsatisfactory control of the coolant flow by the thermostatic expansion valves.
In order to obtain an even coolant flow at the inlet of the flow divider, relatively long steadying sections are often provided immediately upstream of the inlet. Vertically rising or falling steadying sections have proven to be especially advantageous.
However, under cramped mounting conditions, such as exist in motor vehicles, it is not possible to use such steadying sections. Because of sealing problems at the evaporator housings, it is also not possible, in this situation, to mount the flow divider directly behind the expansion valve (which would result in a somewhat more satisfactory division of the coolant flow). It is often necessary to use a bent or spiral-shaped connecting means between the expansion valve and the flow divider. Under these conditions, because of the different inertia of the gaseous coolant and the liquid coolant, there is a separation of the two phases and a development of rotational flows. Since these asymmetric rotational flows are divided into circular segments by the flow divider, certain sets of evaporator pipes are flooded with liquid coolant, while other sets are predominantly filled with gaseous coolant and contribute little to the heat exchange process. The efficiency of the evaporating apparatus is therefore severely impaired. Additionally, as noted above, the performance of the evaporating apparatus is further impaired due to the control characteristics of the thermostatic expansion valves under these conditions.
It is an object of this invention to provide an evaporator apparatus of the above-mentioned type in which a bent or spiral-shaped connecting means can be used between the expansion valve and the flow divider, while nevertheless providing a homogeneous two-phase flow at the inlet of the flow divider.
This object is achieved in an evaporating apparatus of the above type by providing a mixing element arranged directly in front (i.e., upstream) of the flow divider in the coolant flow path. The flow cross-section of the mixing element expands suddenly in the direction of the coolant flow. Due to the expansion of the flow cross-section, a rapid expansion of the two-phase flow (i.e., the gaseous coolant and the liquid coolant), is achieved so that these two phases are mixed together. The result is a flow which is a homogeneous mixture of liquid and gaseous coolant. When the flow divider divides this homogeneous liquid/gas flow into different circular segments, it is ensured that each set of evaporator pipes receives a homogeneous mixture of equal characteristics. This improves the efficiency, as well as the control, of the evaporating apparatus.
The point at which the two-phase flow enters into the mixing element is at a slightly reduced pressure level. Thus, mixing takes place adiabatically (i.e., without a supply of heat from the environment). Therefore, the efficiency of the evaporator apparatus is not reduced by the installation of a mixing element according to this invention. Additionally, the slight drop in pressure accross the mixing element does not impair the function or the efficiency of the expansion valves. Finally, no special manufacturing conditions or tolerances are required--with the exception of the general requirements for cooling equipment, regarding resistance to pressure, tightness, and cleanliness--so that the evaporator apparatus of the present invention can be manufactured at costs that are not much higher than those of known evaporator apparatuses.
In a preferred embodiment, the mixing element comprises a cylinder-shaped mixing cell, the diameter d.sub.W of which is larger than the diameter d.sub.E of the entry opening for the coolant flow. It has been found to be especially advantageous if the ratio d.sub.E /d.sub.W is at least 1/2 and no more than 2/3.
It is preferred to arrange the entry opening for the coolant flow on the cylinder shell (i.e., sidewall) of the mixing cell. In this case, the rapid expansion of the two-phase flow and the mixing of the gaseous and liquid coolant is further assisted by the impact of the liquid particles on the cylinder wall opposite the entry opening.
The flow divider may be placed directly at the opening of the mixing cell. Thus, no additional connecting means is required between the mixing cell and flow divider. It is preferred to also locate the opening for the flow divider on the cylinder shell of the mixing cell.
The mixing element may also be constructed differently. It may, for example, take the form of an insert having a cross-section that at first tapers in the flow direction. After this tapering, the cross-section expands again in the flow direction, resulting in the above-mentioned mixing to produce the homogeneous liquid/gas flow. Since the tapering occurs first, this insert can be installed in a pipe or between two pipes which have constant diameters. A change in the cross-section of these pipes is therefore not required.
It is also possible to fasten a tapered insert to the inside of one or several pipes, such that the expansion of the flow cross-section is located at the downstream end of the tapered part of the insert where the flow discharges into the pipe. In this case, the insert is very simple in construction and can be installed in pipes having the same diameters. It is especially preferred to provide this insert with a collar which extend around its circumference and which rests against the ends of two pipes (i.e., the collar is held between the opposing pipe ends). For increased sealing effect, one of these pipes may additionally be provided with an enlarged end which overlaps the collar.
In yet another embodiment, the mixing element may simply consist of a screen. This screen, in a manner similar to that of the collar mentioned above, may be fastened between the ends of two pipes.
Another application of the invention is in evaporator apparatuses where the coolant connections from the flow divider to the pipe sets must be located on a certain side of the apparatus. Since the suction pipes (i.e., the pipes accepting the discharge of the gaseous coolant) are located on the downstream end of the evaporator apparatus, each evaporator pipe set has an uneven number of pipes. Frequently, the number of evaporator sets (i.e., the number of coolant inputs) is even, requiring an even number of pipes in the evaporator assembly, because the even number of evaporator sets multiplied by the uneven number of pipes per set results in an even number. However, the evaporator assemblies are often constructed in such way that they contain an uneven number of pipes. In this case, one pipe remains empty.
This is best explained by means of an example: It is assumed that an evaporator assembly is nine pipes wide and five pipes deep, for a total of 45 pipes. The coolant input is of a quadruple design, resulting in four evaporator sets. Thus, each of these evaporator sets has 11 pipes, so that 4.times.11=44 pipes are required. The 45th pipe remains empty.
Because of the high gas velocities and the resulting high pressure drops, the suction pipe to the compressor cannot be connected, via the empty pipe in the evaporator apparatus, to the connection side of the evaporator. However, by means of the present invention, it becomes possible to use this empty pipe as the connecting means between the expansion valve and the flow divider. The expansion valve is therefore located on the side of the evaporator assembly that faces away from the connection side, with the empty pipe connecting this expansion valve to the connection side. On the connection side, a mixing element according to the present invention is provided to ensure a homogeneous liquid/gas flow to the inlet of the flow divider. This arrangement does not reduce the performance of the evaporator apparatus. By so utilizing the empty pipe in the evaporator assembly, the need for routing an evaporating apparatus connection means around the evaporator assembly is avoided. The routing of such connection means can be especially difficult, or even impossible, in cramped spaces.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purposes of illustration only, a preferred embodiment in accordance with the present intention.